This invention relates to spatial light modulators, and particularly though not exclusively to the type of spatial light modulator which has become known as a deformable mirror device or DMD.
For a background description of several types of spatial light modulator reference may be made to HUIGNARD, J. P., "Spatial Light Modulators and their Applications, J. Optics (Paris), 1987, Vol. 18, No. 4, pp 181-186. By way of example another type of spatial light modulator is described in THOMAS, R. N. et. el. "The Mirror-Matrix Tube: A Novel Light Valve for Projection Displays", IEEE Transactions on Electron Devices, Vol. ED-22, No. 9, September 1975, pp 765-775.
The deformable mirror device is a particular type of spatial light modulator and comprises a micro-mechanical array of electronically addressable mirror elements, the elements corresponding to pixels. Each pixel mirror element is capable of mechanical movement in response to an electical input. Such movement is in practice more often a deflection rather than a deformation but the term deformable mirror device has now become accepted as the description of this class of devices. They may be digitally addressed in which case they can be referred to as digital mirror devices. The expression DMD covers all of these.
For a description of current DMD technology reference is made to HORNBECK, L. J., "Deformable-Mirror Spatial Light Modulators" Proc. SPIE Critical Reviews Series, Vol. 1150, 6-11 Aug. 1989, San Diego, Calif., U.S.A., pp 86-102. This paper contains many references to earlier work and attention is drawn particularly to references 3, 9, 14 and 23 of that paper. Further details of the construction of the devices is found in BOYSEL, R. M., "A 128 .times.128 frame-addressed deformable mirror spatial light modulator" Optical Engineering, Vol. 30, No. 9, September 1991, pages 1422-1427. Attention is also drawn to reference 1 in that paper which is an earlier publication by Boysel et al. It has been proposed that DMDs should be usable as projection displays, see e.g. HORNBECK, L. J., et al., "Deformable Mirror Projection Display", SID 80 Digest, pp 228-229 (Abstract of presentation delivered Jul. 20, 1980 at SID Symposium), and U.S. Pat. No. 4,680,579.
The construction and manufacture of DMDs is further described in U.S. Pat. Nos. 4,615,595 and 4,566,935 and European Patent Application EP-A-0 391 529, all of Texas Instruments Incorporated.
The following description assumes a knowledge of the above-noted prior documents, all of which are hereby incorporated by reference.
In summary, the DMD is a solid state spatial light modulator that consists of an array of movable mirrors. These mirrors are suspended on high compliance torsional hinges. The physics of the mirrors are such that they can either be set to deflect light on to a screen or away from it. The DMD is therefore a binary device. The modulation of light intensity required to create full colour pictures is achieved by varying the time for which each mirror is switched on, in proportion to the value of the pixel at the given mirror position.
The current device consists of mirrors approximately 20 .mu.m square, separated by a space of 2-3 .mu.m. As noted above, it has been proposed to use a DMD in a projection television or video display. For the conventional (as opposed to HDTV) television signal version, there will be 576 lines of mirrors.
The problem with projection displays has always been to get enough light onto the screen. The mirror area therefore should be of maximum possible size.
However, with existing devices that seek to project television signals onto a large screen, the magnification involved can make the picture line structure subjectively visible. This can be overcome by line interpolation, but as well as being more complicated and frequently less than satisfactory, such process usually requires the display device to have twice as many active elements, doubling the cost and increasing complexity. This is particularly so with the DMD, which requires a complicated electronic interface to supply it with data in the required time.